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CWG1699 Does befriending a class befriend its friends? #1573
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Activity
EWG Discussed this on Monday in Varna, and the following poll was taken:
EWG agrees with the 'first' CWG proposed solution from the November '21 rationale on CWG 1699, that friend functions defined in a class get access to the private/protected members of the class, however friend functions NOT defined in the class definition do not.
SF F N A SA 2 14 5 1 1 However, further discussion lead to further explanation of the inconsistencies, and EWG determined that a paper (written by @jfbastien, Jeff Snyder, et al) explaining the following examples should be presented later in the week:
class c { class n {}; friend struct s; }; c::n g() { // #4 // 'n' accessible here? } struct s { friend class c::n; // #1 friend c::n g(); friend c::n h(); friend void f() { c::n(); } // #3 (EDG/MSVC Reject, Clang/GCC Accept) }; c::n h() { // #5 // 'n' accessible here? } 3 disallowed 3 &4 allowed, same meaning (no matter where 4 is) 3 & 5 allowed, same meaning (5 only allowed if 'after' friendship)Sad paper is encouraged to consider templates (class template, function template, and member function templates), as well as modules.
Note that there's a similar issue that is usually considered to be part of CWG1699: if
fis a friend ofT, doesfalso have access to private members of the class thatTis nested withn?class A { static int x; class B { friend int f() { return A::x; } // OK? friend int g(); }; }; int g() { return A::x; } // OK?It would be good if the paper would also discuss this case.
The discussion included a request to discuss templates in modules as well.
EWG St. Louis: @villevoutilainen volunteered to write a paper.
Reacted by Brian Bi
A friend declaration is a member-declaration, but it is not clear how far the granting of friendship goes in a friend declaration. For example:
In particular, if a friend function is defined inside the class definition, as in #3, does its definition have access to the private and protected members of the befriending class? Implementations vary on this point.
See CWG1699 for the full discussion.